In recent years waveguides of rare-earth doped glass have proved to be particularly attractive for optical communications, since doping gives the guide active characteristics, permitting its use as a coherent source, as an amplifier, etc. Not only active waveguides in fiber form, but also planar or strip waveguides, commonly referred to as "integrated optical waveguides", have been produced. The transmission media most widely used in optical telecommunications systems are monomode fibers and the devices to be interfaced with such media should also be monomode. Monomode integrated active guides are hence of special interest.
The use of ion exchange techniques, commonly utilized to fabricate conventional glass or silica guides, would be desirable also to manufacture strip monomode active guides. These techniques in fact allow fabrication of low attenuation guides, with even complex geometries, in a relatively simple way, are relatively inexpensive and give reproducible results, all of which are important for industrial manufacture. However, in the manufacture of conventional guides, ion exchange concerns monovalent ions, which have high mobility even at relatively low temperatures, whereas manufacture of active guides requires substituting ions in the vitreous matrix with rare earth ions, which are trivalent. A ion exchange of this kind is very difficult to achieve. In fact at the temperatures (300.degree.-500.degree. C.) which are generally used for fabricating conventional optical guides by ion exchange and which are not detrimental to the vitreous matrix of the substrate, rare earth ions have very low mobility, so that the concentrations necessary for active guide fabrication cannot be achieved in reasonable times.
For this reason, the techniques commonly used to fabricate strip active optical guides generally start from a vitreous substrate which is already doped with rare earths and obtain the guiding regions on this substrate by exchange between alkaline ions in the glass and monovalent ions intended to raise the refractive index in the region involved in the exchange (e.g. exchange between Na+ or Li+ glass ions and K+, Ag+ ions) as for a conventional guide. This technique has been described in the paper entitled "ion-exchanged rare-earth doped waveguides" presented by S. I. Najafi et al. at the International Congress on Glasses for Optoelectronics, Paris, 1989 and published in SPIE Proceedings, Vol. 1128. pages 142 and ff. Yet this method is rather expensive, since it requires doping the whole substrate and not only the guiding regions, and therefore it requires the use of large quantities of rare earth salts, which are expensive per se.
A method of fabricating strip active guides allowing rare-earth ion introduction only in the active regions is described by T. Kitagawa et al. in the article entitled "Guided-wave laser based on herbium-doped silica planar lightwave circuit", Electronics Letters, Vol. 27, No. 4, 14 Feb. 1991. In accordance with this method, a core formed of a P.sub.2 O.sub.5 - SiO.sub.2 layer into which herbium ions have been introduced is deposited on a silica substrate by flame hydrolysis deposition. The resulting core is given the geometric structure desired by reactive ion etching techniques and it is covered with a silica overcladding still by a flame deposition technique (FHD). However said techniques produce guides with relatively high attenuation.